1. Field of the Invention
This invention relates in general to forced air furnaces of the type used for residential and office building heating, and more particularly relates to systems for controlling operation of forced air furnaces.
2. Description of the Prior Art
The prior art commercial and residential forced air heating systems employ a heat exchanger which separates the furnace combustion chamber from the path of air leading to rooms or other spaces in the building which are to be heated. During normal operation, the heat exchanger provides a barrier preventing escape of the combustion products into the air flow. Cracks or other defects in the heat exchanger can release the combustion products into the air flow and building airspace. Components of the combustion products which are poisonous to humans, principally CO gas, is detrimental to health, and in many cases causes death to the building occupants. Because CO gas is colorless and odorless, the occupants are oftentimes unaware of the problem and can become asphyxiated. Among the prior art attempts to provide occupants with warning of abnormal furnace operation is U.S. Pat. No. 4,171,944 to Hirschmann. The patent provides a smoke detector electrically connected to shut a furnace down when excessive smoke escapes into the room containing the furnace. U.S. Pat. No. Re. 30,936 to Kmetz provides a temperature-dependent control system which senses air temperature in exhaust ducting to cut out the burners of a furnace when the exhaust gases are too hot. U.S. Pat. No. 3,582,247 to Faure provides a system for shutting a boiler down when leaks of combustible fluid are detected. U.S. Pat. No. 4,715,214 to Tveter detects leakage from a pressurized system by sensing and calculating differences in gas pressure values. U.S. Pat. No. 4,221,206 to Haas relates to automobile systems and provides a system for shutting off an internal combustion engine responsive to carbon monoxide detectors.